Despite substantial lifestyle changes such as in diet or use of complementary and alternative medicine (CAM) among women with breast cancer, few studies have examined whether such factors improve prognosis. These factors may also influence quality of life, which may in turn influence prognosis. How these factors influence prognosis may depend in part on molecular characteristics such as genetic polymorphisms that influence oxidative damage or DNA repair, or aberrant DNA methylation which influences gene expression. These markers may themselves influence prognosis or interact with conventional therapies. We propose to address these gaps in knowledge by establishing the largest prospective cohort study of women with breast cancer to date. We will enroll at least 5,021 women with breast cancer from Kaiser Permanente of Northern California (KPNC). With extremely rapid case ascertainment through computerized pathology reports, we can identify cases of breast cancer as they are confirmed histologically, minimizing survival bias. We will interview and send questionnaires to participants, and extract data from medical charts and KPNC databases. Blood samples will be collected prior to treatment to characterize genetic polymorphisms, and tumor specimens will be obtained to examine aberrant DNA methylation. Blood samples and breast tumor DNAs will also be banked for future use. This resource will enable study of the effects on recurrence and survival of: 1) lifestyle factors, including diet, physical activity, use of CAM, and quality of life; and, 2) host and tumor molecular characteristics, including genetic polymorphisms (e.g., those involved in: cyclophosphamide (CYP3A4, GSTP1, GSTA1) or tamoxifen metabolism (SULTIA1); protection against oxidative damage (MnSOD, CAT, GPX1, GSTM1, GSTT1); and DNA repair (XRCC1, LIG4, XRCC3, XPD, ERCC1, APE1)), and aberrant DNA methylation of genes in breast tumors (BRCA1, P161NK4a, E-Cadherin, glypican3, DUTT1, HIC1, TSLC1, DAP-kinase, GSTP1). Using proportional hazards regression, we will examine associations of lifestyle factors and molecular markers on risk of recurrence and mortality; we estimate at least 599 recurrences and 331 deaths during the 5-year funding period. For survival, we will have power to detect a relative hazard of 1.64 comparing upper to lower quartiles of continuous exposures such as nutrient intake, and a relative hazard of 159 for an exposure with prevalence of 0.10. This study will provide some of the first information on these risk factors and breast cancer prognosis.